Postseismic deformation modeling of the 2001 Mw7.8 Kokoxili, western China earthquake constrained using InSAR time-series data

We processed the ERS data and the ASAR data of ESA for detecting the post-seismic deformation of the 2001 Mw7.8 Kokoxili earthquake (also known as the Kunlunshan earthquake). Because the earthquake happened at the time of the ERS instrument was fading out, some of the important post-seismic signals were not captured by the ERS satellite. Nevertheless, we still obtained valuable data immediately after the earthquake, providing 35-day post-seismic deformation of the earthquake in the range direction of radar sensor. We processed and corrected these data, under constraints using sparsely distributed GPS data measured during the same time period. The results show as large as 3~5 cm postseismic displacements in the LOS direction. Since 2003, the new Envisat satellite (using ASAR sensor) has replaced its predecessor, and we requested its radar data of the Kokoxili area regularly for InSAR detection of the post-seismic deformation signals. The time frame of the data is from April, 2003 to April, 2008 and the spatial coverage is ~88°E - ~96°E, and ~34°N - ~38°N, with 5 descending tracks. We processed the data using the small baseline subset (SBAS) method, and the largest baseline is shorter than 300 meters. The time series is referenced to the earliest acquisition of the radar data. We then fit a time decay of the signals incorporating the early stage GPS data, to supplement the acquisition gap of SAR data between 2002~2003. Our results show that: (a) The postseismic deformation follows the overall coseismic deformation pattern, trending westward for sites located north of the fault and eastward for sites south of the fault, respectively. (b) The deformation pattern is remarkably asymmetric across the fault, the southern sites move significantly faster than the northern sites of comparable distances from the fault. (c) The time-dependent postseismic deformation data fit nicely to an exponential decay function, with the decay time of ~0.3 year. We model the postseismic deformation of the Kokoxili earthquake to understand the fault zone rheology and the crust and upper mantle structure. We invert kinematically the early stage deformation with an afterslip model in homogenous half space first. We then model the postseismic deformation as the result of rheological response to the coseismic stress change in a layered visco-elastic media. Our result shows that a 20-km thick lower-crust with a viscosity of 5x1017 Pas provides the best fit to the InSAR time-series data. We are using a finite element code to model asymmetric postseismic deformation produced due to lateral change of the media properties. The result will be presented at the meeting.